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Fundamentals of silicene
Published in Klaus D. Sattler, Silicon Nanomaterials Sourcebook, 2017
Gian G. Guzmán-Verri, Lok C. Lew Yan Voon, Morten Willatzen
The first report of the fabrication of a crystalline Si monolayer sheet was possibly in 2006.9 In that paper, the authors stated that “Although many researchers have attempted to prepare two-dimensional Si sheets, there have been no reports of a successful fabrication of Si monolayer sheets.” They reported the fabrication of the nanosheets by chemical exfoliation of magnesium-doped calcium disilicide. Given that the bonding between Ca and Si is ionic and charged, Mg was used to reduce the charge on the Si. The doped material was then immersed in a solution of propylamine hydrochloride which deintercalated the Ca ions. They concluded that the sheets were oxidized and magnesium-doped. From high-resolution atomic-force microscopy (AFM) and transmission electron microscopy (TEM) and from electron diffraction, they deduced that the sheets are single crystalline and (111) oriented with a lattice constant of 0.82 nm and thickness of a slightly squashed Si(111) plane. They observed the sheets to be almost transparent and having lateral dimensions in the range 200–500 nm. They did find the Si nanosheets to oxidize easily and, therefore, aggregate, making it difficult to disperse in a solvent.
Hexa ↔ tetra silicene crystal–crystal phase transition
Published in Philosophical Magazine, 2020
Vo Van Hoang, Nguyen Hoang Giang, Vladimir Bubanja
Hexa-silicene, a monatomic sheet of silicon atoms, shares many of the outstanding properties of graphene, and has the great potential for applications due to the compatibility with current silicon-based nanoelectronics. Early considerations of silicon aromatic compounds, based on the total energy calculations within the local-density functional approach, predicted the corrugation of infinite 2D silicon lattice [1]. Similar conclusions resulted from the density functional theory calculations, which demonstrated a stable honeycomb structure of Si with buckling of 0.44 Å and a bond length of 2.25 Å [2]. Calculations of the phonon modes showed that the flat silicene is metastable [2]. The buckled form has lower energy by 30 meV/atom and has a lower binding energy by 0.6 eV/atom than the bulk Si [3]. As in the case of graphene, the tight-binding Hamiltonian approach showed the presence of Dirac cones [4]. Similar results were obtained by ab initio calculations [3], and were experimentally confirmed with silicene on Ag(111) [5]. Layered Si crystals composed of 2D sheets, held together by van der Waals forces, are not known to exist. Therefore, mechanical exfoliation that has been employed in the case of graphene is unlikely to be developed for silicene. Instead, chemical exfoliation of calcium disilicide was used to produce functionalised silicene (see [6–9] and references therein). Molecular beam epitaxy under ultra-high vacuum was successfully employed to deposit silicene on silver substrates [10–12]. While the growth of silicene has also been reported on ZrB2, Ir, ZrC and MoS2, silver seems to be an ideal substrate due to the compatibility of its crystal structure and lattice commensuration with the freestanding form of silicene. On the other hand, there are still several unresolved issues related to growth conditions that result in a variety of silicene superstructures formed in the deposition process [13,14], as well as difficulties in the interpretation of the band structure near the Dirac point due to the hybridisation between the Si and Ag electronic states (see [10,15] and references therein). Given the similarity with graphene, many of the same fundamental properties and potential applications of graphene have been considered for silicene. Among others, these included quantum spin Hall effect [16], quantum anomalous Hall effect [17], valleytronics [18], spintronics [19], superconductivity [20,21], band gap engineering [22], field effect transistor (FET) [23,24], topological insulator FET [25], thermoelectric effects [26], gas sensing [27], energy storage [28], thermal conductivity [29], as well as structure and electronic properties of the double Si layer [30] or freezing transition from liquid Si to quasi-2D bilayer Si in a slit nanopore [31].